Protecting a Stationary Vessel from Encroaching Ice

ABSTRACT

Techniques are provided for protecting a stationary vessel from encroaching ice. In an example, a system has a subsea mount disposed on a seafloor and a thruster disposed on the subsea mount under a water surface. The thruster is configured to destabilize a water column under the encroaching ice.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of U.S. Provisional PatentApplication 62/087,504, filed Dec. 4, 2014, entitled PROTECTING ASTATIONARY VESSEL FROM ENCROACHING ICE, the entirety of which isincorporated by reference herein.

FIELD

The present techniques are directed to protecting stationary vesselsfrom sea ice. More specifically, the present techniques are for breakingsea ice drifting towards a stationary vessel.

BACKGROUND

This section is intended to introduce various aspects of the art, whichmay be associated with exemplary embodiments of the present techniques.This discussion is believed to assist in providing a framework tofacilitate a better understanding of particular aspects of the presenttechniques. Accordingly, it should be understood that this sectionshould be read in this light, and not necessarily as admissions of priorart.

Arctic drilling attention has recently transcended shallow waters, e.g.,less than about 100 meters (m), into deep waters, e.g., greater thanabout 100 meters. This, however, multiplies the challenges as arctic anddeep-water frontiers are now merged. The challenges generally resultfrom problems in stationkeeping, which is the ability of a vessel tohold a position over a subsea location, such as a well.

Station keeping is often important to prevent stress on drilling risersand lines that run from a vessel to the seafloor. Station keeping can beperformed passively, such as by mooring lines, dynamically usingpropulsion systems, or a combination of the two. In arctic environments,station keeping can be challenged by sea ice floes. While fixedplatforms are in direct contact with the seafloor and can withstandforces up to tens of thousands of tonnes, floating platforms areanchored to the seafloor via mooring systems with the capacity towithstand forces in the range of 1,000-2,000 tonnes.

While subsea developments may become a viable concept for arcticdeep-water development, some operations still need to be conducted atthe surface, such as drilling a subsea well or loading crude from subseastorage, among others. With conventional technology, these surfaceoperations would have to rely on open water season. However, in manyarctic locations, an open water season can be limited, highly variable,or, in some years, non-existent. Accordingly, systems to protectplatforms and other vessels from drifting ice may be useful. Extendingthe open water season will depend on ice breakers, which may be limitedin number and costly to operate.

SUMMARY

An exemplary embodiment provides a system for protecting a stationaryvessel from encroaching ice. The system includes a subsea mount disposedon a seafloor and a thruster disposed on the subsea mount under a watersurface. The thruster is configured to destabilize a water column underthe encroaching ice.

Another exemplary embodiment provides a method for protecting a seasurface location from encroaching ice. The method includes detectingencroaching ice and activating a seafloor mounted thruster, wherein thethruster destabilizes the water column below the ice.

Another exemplary embodiment provides a method for producinghydrocarbons. The method includes positioning a vessel at a location ona sea surface. A thruster attached to a mount on a seafloor ispositioned proximate to the location. Hydrocarbons are produced from awell using the vessel. Ice that is encroaching on the vessel isdetected. The thruster is activated to destabilize the water columnbelow the ice.

DESCRIPTION OF THE DRAWINGS

The advantages of the present techniques are better understood byreferring to the following detailed description and the attacheddrawings, in which:

FIG. 1 is a schematic diagram of the use of anchored thrusters toprotect a stationary vessel from encroaching ice floes;

FIG. 2 is a schematic diagram showing thrusters breaking an encroachingice floe;

FIG. 3 is a schematic diagram of a single thruster destabilizing a watercolumn under an encroaching ice sheet;

FIG. 4 is a drawing of a single thruster that may be used for breakingup an encroaching ice floe;

FIG. 5 is a schematic diagram of a drilling location showing theplacement of sensors and thrusters that may be used to protect a vessel;

FIG. 6 is a process flow diagram of a method for producing hydrocarbonswhile using thrusters to break up ice floes before they reach a drillinglocation; and

FIG. 7 is a process flow diagram of a method for using thrusters tobreak up ice floes before they reach a surface location.

DETAILED DESCRIPTION

In the following detailed description section, specific embodiments ofthe present techniques are described. However, to the extent that thefollowing description is specific to a particular embodiment or aparticular use of the present techniques, this is intended to be forexemplary purposes only and simply provides a description of theexemplary embodiments. Accordingly, the techniques are not limited tothe specific embodiments described below, but rather, include allalternatives, modifications, and equivalents falling within the truespirit and scope of the appended claims.

As discussed above, protecting a vessel or vessels at a sea surfacelocation from encroaching ice may be problematic, especially in deepwater operations. Accordingly, embodiments described herein provideprotection from ice encroachment for a location at the surface.Thrusters are mounted to locations along the seafloor and used todestabilize the water column under ice floes. Destabilizing the watercolumn may break up the ice floes into fragments that do not apply asmuch pressure to a vessel. In some embodiments, the thruster may be usedto steer larger ice fragments, such as ice ridges or icebergs, away fromthe vessel.

The thrusters may be mounted in arcs upstream of the vessel, e.g., inthe general direction of encroaching ice floes, to break up or divertice floes before they reach the vessel. In some embodiments, the subseamount includes rails (or tracks) mounted to the seafloor. The thrustersmay be mounted along the rails, allowing the thrusters to berepositioned to be more effective. For example, the rails may be mountedto mooring pilings, such as suction pilings, used to moor the ship inplace. Combinations of these techniques can be used to make multiplelayers of protection. For example, fixed thrusters may be mounted in anarc upstream of the vessel, while a rail may be mounted proximate to thefixed thrusters, allowing additional thrusters to be moved into place asneeded. Similarly, multiple rails with mounted thrusters can be nestedaround a location to protect vessels by allowing multiple thrusters oneach of the rails to be moved into place as needed.

The thrusters can be controlled and powered from the vessel beingprotected or may have a separate power system, control system, or both.In some embodiments, the thrusters may have sensors to detect objects,such as sea life, vessels, ice floes, and the like. The detection of theobjects may be used to adjust the depth of the thruster in the watercolumn, to power the thruster down, or both, to avoid collisions anddamage.

FIG. 1 is a schematic diagram of the use of anchored thrusters 102 toprotect a stationary vessel 104 from encroaching ice floes 106. Thevessel 104 may be a drillship, used to drill a well 108 to a reservoirlocated below the seafloor 110. However, as discussed herein, any numberof other vessels may be protected using the current techniques,including tankers, service vessels, floating storage, offloading, andproduction (FSOP) vessels, and the like. If the vessel 104 is in deepwater, for example, greater than about 100 meters in depth, it may notbe in direct contact with the seafloor 110. In this example, stationkeeping is important to protect equipment and piping that extendsbetween the vessel 104 and the seafloor 110, such as a drilling riser112.

The station keeping can be performed using mooring lines 114, forexample, coupled to a piling 116 embedded in the seafloor 110. Thepiling 116 may be driven into the seafloor or may be a suction piling.Suction pilings are pulled into the soft surface of the seafloor byplacing an open bottom of the piling in contact with the seafloor andthen pumping water out of a chamber located at the top of the piling. Insome embodiments, station keeping can be performed by azimuthingthrusters 118 on the vessel 104. Combinations of these techniques mayalso be used, for example, using a mooring line 114 to generally holdthe vessel 104 in place while the azimuthing thrusters 118 hold tensionon the mooring line 118 and keep the vessel 104 from lateral motions.

Encroaching ice floes 106, or ice ridges 107, may approach the vessel104 and interfere with the station keeping, for example, by forcing thevessel 104 to disconnect the drilling riser 112 from the well 108, pullit up, and move out of the way. The use of the thrusters 102 in a watercolumn 120 can mitigate problems with the ice floes 106. The thrusters102 may be disposed on the subsea mount by attaching the thrusters 102to the mount using tethers 122. The subsea mount may include a frame andone or more pilings. The frame, called a template 124, may be attachedto pilings 126 set into the seafloor 110. The pilings 126 may be driveninto the seafloor 110 or may be suction pilings. In other embodiments,the thruster may be attached to a piling 116 used for mooring thestationary vessel 104. For example, mooring lines for a drilling vessel104 may be attached to a series of suction pilings located around thevessel. Each of the suction pilings may also be used as attachmentpoints for anchoring thrusters 102. As discussed herein, the pilings 116may also be used as anchor points for rails that are used as attachmentpoints for anchoring the thrusters 102. This would allow the thrustersto be moved to positions that are between encroaching ice floes 106 andthe stationary vessel 104.

A power and control cable 128 may be attached from the vessel 104 toeach tether 122 to provide power to the thruster 102 attached to thattether 122. In some embodiments, a subsea generator may be used toprovide power for a thruster 102. Further, power may be provided to thethrusters 102 from an on-shore generating station, depending on thelocation of the field relative to the shore, e.g., within about 50 milesof the shore or less.

The thrusters 102 can be made from any number of marine propulsionunits, such as tunnel thrusters available from Rolls-Royce PLC ofLondon, England, and Thrustmaster of Houston, Tex., USA, among others.The selection of the sizes for the thrusters 102 may depend on thelocation of the vessel 104 and the likely type of sea ice to beencountered, such as first year, second year, etc. In some embodiments,the thrusters 102 may be about 180 kilowatts (kW) to about 1 megawatt(MW) in power generation, while in other areas, the thrusters 102 may beabout 3 to about 8 MW.

As the thrusters 102 may take water in at the top and eject it from thebottom, the tether 122 will remain in tension. Further, the tether 122may be coiled to allow the thruster 102 to move vertically in a watercolumn 120 without the tether 122 developing slack. A thruster 102 maychange its vertical position along a water column by adjusting itsbuoyancy. This allows the thrusters 102 to be useful for various waterdepths and avoid various sea ice keel depths.

In some embodiments, a thruster 102 may be configured to allow areversal of flow, e.g., from the bottom of the thruster 102 and out thetop. This may allow the thruster 102 to deflect ice floes 106 or iceridges 107 away from the vessel 104 when they are too large to break up.In this case, the thruster 102 would not be mounted to the template 124by a flexible tether 122, but would, instead, be disposed on the subseamount by attaching the thruster with a bar or other rigid part. The barmay be hinged at the bottom to allow the thruster 102 to be lowered in awater column 120, for example, to avoid a collision.

FIG. 2 is a schematic diagram showing thrusters 102 breaking anencroaching ice floe 202. Like numbered items are as described withrespect to FIG. 1. In this example, the thrusters 102 are configured topull water in through the top 204 and eject the water from the bottom206, as indicated by arrows. Removing water from underneath the ice floe202 can cause it to collapse under its own weight, as indicated by thecracks 208 forming in the ice floe 202. The fragments 210 formed by thecollapse may be too small to cause problems with the station keeping ofthe vessel.

The tethers 122 may be used to set the vertical depth 212 at which thethrusters 102 sit under the ocean surface 214. The depth 212 may beselected by a combination of factors, including the expected size of theice floes 202, the clearance needed for vessels operating in the area,and the size of the thrusters 102. In various embodiments, the thrusters102 may be set to be about 5 meters to about 30 meters below thesurface.

FIG. 3 is a schematic diagram of a single thruster 102 destabilizing awater column 300 under an encroaching ice sheet 302. Like numbered itemsare as described with respect to FIGS. 1 and 2. As described herein, thethruster 102 may take inlet water 304 in through the top 204 and ejectoutlet water 306 through the bottom 206. The flow may lower the pressurein the water column 300 under the ice sheet 302, resulting in a loss ofsupport for the ice sheet 302. The ice sheet 302 may then form fractures308 and, as the ice sheet moves over the unstable water column 300, thefractures 308 may be propagated through the ice sheet 302, breaking itinto smaller fragments.

The water ejected from the bottom 206 of the thruster 102 may be flowedthrough a grate or other structure to divert the flow out to the side.This may be useful for decreasing scouring of the ocean bottom below thethruster 102, which could lead to loss of integrity of the supportingpiles 126.

Although the thrusters 102 may cause ice sheets 302 and ice floes 202 tobreak up, they may also be useful against larger ice structures, such asice ridges. For example, the flow from the thrusters 102 may be used tochange a path of an ice ridge, diverting it away from vessels. This maybe caused by the swirling motion and indentation that would form in thesurface above the thruster 102, e.g., a whirlpool. The control systemmay be used to determine how to use the thrusters 102 to divert thelarger ice structures.

FIG. 4 is a drawing of a single thruster 102 that may be used forbreaking up an encroaching ice floe. Like numbered items are asdescribed with respect to FIGS. 1-3. As described herein, the thruster102 is coupled to a tether 122, which may also provide power andcontrol. The inlet water 304 is pulled in the open top 204 by thrusterblade 402 that is turned by a motor 404. The motor 404 may be anelectric motor or may be a hydraulic engine, for example, powered by asea water flow from a pump on a vessel. The outlet water 306 is ejectedthrough the bottom 206 of the thruster 102. As described herein, a grate406 may be used to divert the flow of the outlet water 306 away fromdirect impact with the seafloor, lowering the chance of scouring aroundthe mounting piles or rail.

The thruster 102 may have buoyancy compartments 408, which may be usedto increase or decrease the depth of the thruster 102. The buoyancycompartments 408 may be, for example, hollow vessels that can bepartially or completely filled with air from a compressor on the vessel.The buoyancy compartments 408 may not be effective at adjusting thedepth when the thruster 102 is operating, as the thrust generated maytend to push the thruster 102 closer to the surface of the water.However, the buoyancy compartments 408 may be useful for lowering thethruster 102 to avoid an impact with an object while the thruster 102 ispowered down. Such objects may be detected or identified by any numberof means, such as by a sensor 410 mounted to the thruster 102. When anapproaching object, such as a ship or a marine mammal, among others, isdetected, the thruster 102 may be instructed to shut down, lower itsdepth in the water, or both.

The thruster 102 may be mounted to the tether 122 by a framework 412extending from the tether 122 to the outside shell 414. In addition tothe open top 204 and bottom 206, the outside shell 414 may also haveother openings 416 that may be used to provide differential thrust ifdesired. One or more of the side openings 416 may be configured to movethe thruster 102 to the side, for example, angled to the side. Theamount of thrust provided by these openings 416 may be controlled bylouvers 418 that may be opened or closed by a control signal sent to anoperator 420 from the vessel.

FIG. 5 is a schematic diagram of a drilling location 500 showing theplacement of sensors 502 and thrusters 102 that may be used to protect avessel 104. Like numbered items are as described with respect to FIG. 1.In this example, two sets of thrusters 102 protect the vessel 104. Thefirst set is mounted to a rail 504 to allow the thrusters 102 to berepositioned as needed, as indicated by an arrow 506. A second set ofthrusters 102 is positioned inside the rail 504, for example, in an icedrift direction from the vessel 102. The rail 504 may also decrease thetotal number of thrusters 102 needed by allowing the thrusters 102 to bemoved back and forth across the front of the encroaching ice.

In this example, an ice sheet 510 is moving in a down drift direction512 towards the vessel 104. Further, ice ridges 514 may be embeddedwithin the ice sheet 510. Three zones can be defined around the vessel104 to respond to the ice encroachment. In an outer zone defined by afirst ring 516, incoming ice, such as the ice sheet 510 and ice ridges514, are detected and tracked to determine the actions that may need tobe taken. The outer zone may have a radius of about 10 to 40 kilometers(km) or greater.

A middle zone can be defined by a second ring 518, for example, at aradius of about 2 to 10 km. Sensors 502, such as sonar sensors, may beplaced along the second line 518 to determine the approach of ice.Encroaching ice may also be detected by other means, such as satelliteimagery, optical sensing, or manual sighting. The middle zone may beused as a management zone to deal with the ice. For example, thrusters102 mounted along the rail 504 may be moved circumferentially intoposition to break up the ice sheet 510 into fragments 520 as it crossesthe thrusters 102. The second set of thrusters 102, mounted inside ofthe rail 504, may be used to further break up the ice sheet 510 or todivert at least some of the ice ridges 514. The second set of thrusters102 may also be mounted on a rail to make this operation more efficient.An icebreaker 522 may be used to divert ice ridges 514 that are toolarge to be diverted by the thrusters 102. The use of the thrusters 102to break up the majority of the ice sheet 510 may allow the ice breaker522 to function more efficiently, allowing fewer ice breakers, or evenjust a single ice breaker 522, to protect the vessel 104 from mostencroaching ice.

An inner zone can be defined by a third ring 524. The third ring 524 maybe located at about 1 to 2 km from the vessel 104 and may be placed at adistance that gives the vessel 104 sufficient time to secure and detacha drilling riser from a well, or other subsea connections, in order tomove away from encroaching ice.

FIG. 6 is a process flow diagram of a method 600 for producinghydrocarbons while using thrusters to break up ice before reaching anoperational location, such as a drilling location. The method 600 beginsat block 602 by positioning a vessel at a location on the sea surface.At block 604, a thruster attached to a mount is positioned on theseafloor. At block 606, while hydrocarbons are being produced using thevessel, ice encroaching on the vessel is detected. At block 608, thethruster is activated to destabilize the water column below the ice.

FIG. 7 is a process flow diagram of a method 700 for using thrusters tobreak up ice before reaching a surface location. The method 700 beginsat block 702 by detecting ice that is encroaching on the surfacelocation. At block 704 a thruster attached to a mount on the seafloor isactivated to destabilize the water column below the ice.

While the present techniques may be susceptible to various modificationsand alternative forms, the embodiments discussed above have been shownonly by way of example. However, it should again be understood that thetechniques are not intended to be limited to the particular embodimentsdisclosed herein. Indeed, the present techniques include allalternatives, modifications, and equivalents falling within the truespirit and scope of the appended claims.

What is claimed is:
 1. A system for protecting a stationary vessel fromencroaching ice, comprising: a subsea mount disposed on a seafloor; anda thruster disposed on the subsea mount under a water surface, whereinthe thruster is configured to destabilize a water column under theencroaching ice.
 2. The system of claim 1, comprising a cable from thestationary vessel to the subsea mount, wherein the cable carries power,control signals, or both to the thruster.
 3. The system of claim 1,comprising a sonar sensor configured to detect encroaching ice.
 4. Thesystem of claim 1, wherein the subsea mount comprises a template and oneor more pilings.
 5. The system of claim 4, wherein the one or more pilescomprise a suction pile set into the seafloor.
 6. The system of claim 1,wherein the subsea mount comprises a rail to allow the position of thethruster to be changed.
 7. The system of claim 1, wherein the thrusterpulls water in through the top of the thruster and ejects the waterdownwards towards the seafloor.
 8. The system of claim 1, wherein thethruster pulls water in through the bottom of the thruster and ejectswater upwards towards the water surface.
 9. The system of claim 1,wherein the thruster comprises one or more side openings configured tomove the thruster to the side.
 10. The system of claim 1, wherein thethruster comprises a bottom grate to redirect the water outward from thethruster.
 11. The system of claim 1, wherein the thruster has one ormore buoyancy compartments for adjusting the vertical depth of thethruster.
 12. The system of claim 1, comprising a sensor on the thrusterconfigured to detect the approach of an object and instruct the thrusterto shut down, lower its depth in the water, or both.
 13. The system ofclaim 1, wherein a plurality of subsea mounts are configured to formmultiple rails in arcs.
 14. The system of claim 1, wherein the thrusteris disposed on the subsea mount using a tether which is coiled to allowvertical mobility from the mount.
 15. The system of claim 1, comprisinga subsea generator to provide power for the thruster.
 16. A method forprotecting a sea surface location from encroaching ice, comprising:detecting encroaching ice; and activating a seafloor mounted thruster,wherein the thruster destabilizes the water column below the ice. 17.The method of claim 16, comprising lowering the thruster to avoid impactwith an object.
 18. The method of claim 16, comprising moving thrustersto positions calculated to intersect the encroaching ice.
 19. The methodof claim 16, comprising powering the thrusters from a vessel at the seasurface location.
 20. The method of claim 16, wherein destabilizing thewater column below the ice includes pulling water in through the top ofthe thruster and ejecting the water downwards towards the seafloor. 21.The method of claim 16, wherein destabilizing the water column below theice includes pulling water in through the bottom of the thruster andejecting the water upwards towards the water surface.
 22. A method forproducing hydrocarbons, comprising: positioning a vessel at a locationon a sea surface; positioning a thruster attached to a mount on aseafloor; producing hydrocarbons from a well using the vessel; detectingice that is encroaching on the vessel; and activating the thruster todestabilize the water column below the ice.